Coordination numbers second-order effects

A second-order effect that contributes to the in situ IR spectra consists of change in the interfacial structure of solvent, including the coordination to the surface, solute, and self-organization. Correlation between the structure of solvent and the HL ionic composition and the electrode surface properties is a considerable objective not only in electrochemistry but also in other numerous areas of science and technology dealing with surface modification. A number of systems have been studied to date, including acetonitrile [110, 115, 159, 179], acetone [110, 179], methanol [110, 180], and benzene [110] at a Pt electrode. However, particularly interesting but yet little understood is the most common solvent, water. 

Second-order effects apply to situations where there are bulky groups attached to the donor atom. The donor atom may be small (C, N, O) but it is the repulsions between the buUqr substituent groups that determine how many ligands may pack round the lanthanide ion, not the repulsions between the donor atoms. Ligands of this type are exemplified by bulky amides, notably N(SiMe3)2, as well as the isoelectronic alkyl-CH(SiMe3)2 and bulky alkoxides and aryloxides. Such ligands are associated with compounds with unusually low coordination numbers. 

The same phenomenon appears to exist in the alkylamides [Ln((N(SiMe3)2)3] (Ln = La-Lu) and alkyls [Ln((CH(SiMe3)2)3] (Ln = Y, La-Lu), whichare all believed to have three-coordinate trigonal p3 amidal structures in the sohd state. Here, however, the constant coordination number is probably due to the second-order effects of the bulky ligands this factor may also be the cause of isostructurality in the neopentoxides Ln(ONp)3 (Ln = Sc (12), Y, La-Lu except Pm), where the tetrameric structure [ Ln( 4-ONp)2(ONp) 4] is maintained in toluene solution. ... 

It is possible to consider first the coordination numbers, and then the atomic numbers, or to proceed in the reverse order. If there are more differences in atomic numbers than in coordination numbers the first mode is more effective, while the existence of many atoms of the same element with different coordination number favors the use of the second. Most atoms of a given chemical element have the same coordination number, particularly in organic compounds. If the H-atoms are, however, disregarded, one obtains constitutional formulas which can be indexed quite well by primary consideration of the degrees of the nodes0), i.e., the number of immediate covalent neighbors. 

So, first-order effects determine the coordination number in species such as LnCl and [Ln(H20)9] +, whereas bulky ligands with high second-order steric effects lead to crowding and impose low coordination numbers in [Ln (CH(SiMe3)2 3] and [Ln (N(SiMe3)2 j]. 

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